Pressurized water nuclear reactors comprise a primary circuit inside which water circulates at a very high pressure of the order of 155 bars and at a very high temperature of the order of 310.degree. C.
Some parts of the components situated in the primary circuit of the nuclear reactor are particularly exposed to cracking corrosion in contact with the water of the primary circuit or in contact with external agents, as a result of the temperature prevailing in the primary circuit.
In particular, the component parts comprising a bimetallic connection are exposed to corrosion of this type.
This applies to the regions of welding between the pipes of the primary circuit and the nozzles connecting these pipes to the vessel, which are internally coated with a layer of stainless steel; the passages through the closure head and bottom of the vessel which consist of tubing made of a nickel alloy such as Inconel 600; the regions of welding of these passages to the closure head or bottom of the vessel; and, more generally, all the regions of the primary circuit comprising coatings or welds or made of nickel alloy.
Cracks have been detected in some parts of the primary circuit of the nuclear rector, after the reactor has been operating for some time, and in particular in regions of a certain heterogeneity of composition, such as the regions of welding or the regions coated with a stainless steel or a nickel alloy. Cracks have also been detected in some passages through the closure head or bottom of the vessel, in the regions of welding on the wall of the closure head or of the bottom of the vessel.
When cracks are detected, it is necessary either to repair the damaged region or to replace the component comprising a damaged region.
The repair operations as well as the operations of replacing components may be very costly and require extended shutdowns of the nuclear reactor.
It is therefore desirable to have available processes for preventive treatment of those regions of the primary circuit of a nuclear reactor which are most exposed to cracking when the reactor is in operation.
Until now there has been no known extremely efficient process making it possible to prevent or to limit cracking of the sensitive regions of the primary circuit of a nuclear reactor, and in particular of regions of heterogeneous composition.
Coating processes which are known and generally employed for protecting metal surfaces against corrosion have not been found effective for protecting heterogeneous regions of the primary circuit of a nuclear reactor or for protecting the nickel alloy surfaces of the primary circuit, which exhibit a certain sensitivity to cracking corrosion.
FR-A-2.509.640, discloses a process of manufacturing a bi-metallic part by coating a piece made of structural steel with a harder layer. The coating is obtained by welding so that the piece melts superficially during the coating.
The coating can be produced by means of a semitransferred arc plasma torch. The coating layer is more than 6 mm thick and can be as thick as 16 mm. The coated piece must be heat-treated by tempering or quenching and tempering.
Such a process cannot thus be used for producing a leakproof protective coating on a surface of a component of a nuclear reactor.